Pentasil-zeolites are defined by their structure type, and more specifically by their XRD. ZSM-5 is the trade name of a pentasil-zeolite.
As early as 1967, Argauer and Landolt (U.S. Pat. No. 3,702,886) worked out parameters for the synthesis of pentasilzeolites, particularly those relating to the following molar ratios: EQU OH.sup.- /SiO.sub.2 =0.07-10 EQU SiO.sub.2/ Al.sub.2 O.sub.3 =5-100 EQU H.sub.2 O/SiO.sub.2 =1-240
However, the Argauer and Landolt procedure succeeded in synthesizing a reasonably pure phase ZSM-5 zeolite only if organic amines with a structure-giving function (i.e. template function), such as tetrapropyleneammonium compounds were used.
Subsequent to the publication of the Argauer and Landolt patent, various publications have disclosed methods of conducting the synthesis of pentasil-zeolites without requiring the very expensive, toxic and easily inflammable organic amine templates. Still other subsequent publications have disclosed substitutes for these amines. In addition to their expense, toxicity and flammability, such amines are disfavored because they are subject to thermal decomposition which can destroy the zeolite structure. Further publications have disclosed modifications of the Argauer and Landolt process directed towards improving the reactivity of the SiO.sub.2 and Al.sub.2 O.sub.3 starting materials.
For example, German Offenlegungsschrift 34 02 842 discloses a method wherein a specially aged aluminosilicate (which is still, however, amorphous to X-rays) is used as a nucleating gel. The method avoids the use of organic amine templates.
EP 0 111 748 discloses aluminosilicates having a zeolite structure and methods for their preparation. The zeolite synthesis is carried out without the addition of an organic compound, but in the presence of aluminum phosphate. However, this method results in zeolites which contain phosphate.
Under normal conditions, the synthesis of zeolites without the use of organic compounds proceeds very slowly. Accordingly, there are no large-scale methods that do not use organic templates for the preparation of high-silica aluminosilicates having a pentasil structure (see Synthesis of High-Silica Aluminosilicate Zeolites, by P. A. Jacobs and J. A. Martens, Studies in Surface Science Catalysis 33, 1987, p.143; and Zeolites as Catalysts, Sorbents and Detergent Builders, H. G. Karge and J. Weithamp, Studies in Surface Science Catalysis, 46, 1989, p.654).
It is also known that formation of aluminosilicates, build-up of the zeolite crystal lattice from Sio.sub.4.sup.- and AlO.sub.4.sup.- tetrahedra, nucleation of zeolites, and zeolite crystal growth all take place by way of reversible reactions. These processes depend on chemical equilibria, which can shift in different directions depending on the temperature, the hydrothermal pressure relationships and the concentrations of reactants (e.g. supersaturated or unsaturated). It is desirable to have a reaction, which can be carried out largely under pressureless conditions and to achieve as complete a reaction as possible to the crystalline aluminosilicate, while avoiding, on the one hand, possible secondary phases (such as cristobalite) and, on the other, on amorphous phase.
High temperatures (i.e., temperatures in excess of 200.degree. C.), and consequent high reaction rates, are advantageous for the formation of crystalline aluminosilicate. However, high temperatures (in excess of 200.degree. C.) increase the probability of forming secondary phases.